(1) Field of the Invention
The present invention relates to an internal combustion engine provided with a valve mechanism.
(2) Description of the Related Art
Automobiles have been required to reduce unburned components of fuel, which are included in an exhaust gas. Accordingly, it has been proposed that an automobile is provided with an exhaust gas purifying catalyst disposed in an exhaust pipe to detoxify unburned components in the exhaust gas. To effectively use the exhaust gas purifying catalyst, it is necessary to increase the temperature of the exhaust gas purifying catalyst until it is activated. Thus, in a cold state, e.g., immediately after the start of an automobile, it is difficult to activate the exhaust gas purifying catalyst since the temperature of the exhaust gas is not high.
On the other hand, there has been known a method in which the phase of the opening timing of an intake valve and an exhaust valve is shifted to control the overlap during opening of those valves, so that unburned components in the exhaust gas can be reduced when an engine is cold. According to Japanese Laid-Open Patent Publication (Kokai) No. 2002-155767, the opening timing of an intake valve is advanced from the top dead center of a piston to increase the quantity of an exhaust gas which flows back into an intake port and is combusted again (internal EGR gas), so that unburned fuel in the exhaust gas is combusted, and the temperature of the intake port is increased due to heat of the exhaust gas to facilitate the carburetion of fuel which is injected next so that unburned components in the exhaust gas can be reduced.
However, according to the above conventional method in which the overlap is controlled, if one of an opening timing and a closing timing is determined, the other is uniquely determined. Therefore, if the opening timing of the intake valve is advanced, the closing timing thereof is also advanced, which reduces the quantity of air-fuel mixture which is heated in a cylinder and flows back into the intake port, and inhibits the facilitation of fuel injected next and the combustion of fuel in an air-fuel mixture. Namely, it is impossible to efficiently reduce unburned components of fuel since the opening timing and the closing timing of the intake valve cannot be determined independently of each other.